An oscilloscope is used to acquire, analyze and display electronic signal waveforms. The oscilloscope takes electronic signals and plots their waveforms on a cathode ray tube (CRT) display screen in units of voltage-versus-time. Conventionally, voltage amplitude is plotted along the vertical, that is, the Y axis and time is plotted along the horizontal, that is, the X axis. In the past decade, digital oscilloscopes have been developed. The basic scheme in digital oscilloscope operation is the sampling of a signal followed by analog-to-digital conversion of the voltage values of the acquired samples. The digitized information is then placed in memory and used to create the display of the signal's waveform.
A trigger level is a voltage value which the input signal must exceed before the oscilloscope begins tracking the signal with samples. The oscilloscope is said to be triggered when this level is achieved. Double triggering is an error caused by a so-called false trigger. A false trigger is noise on the input signal which may appear as a trigger voltage level to the triggering circuitry of the oscilloscope. Double triggering can result in the appearance of two superimposed and out-of-phase versions of the same signal on the oscilloscope display screen. To reduce the likelihood of double triggering, hysteresis is introduced into the trigger circuit of the oscilloscope. Fundamentally, hysteresis is just the difference between two voltage levels. The hysteresis defines a first voltage level, known as the arm point, and a second higher voltage level, known as the fire point. Ideally, the input signal must first exceed the arm point and then the fire point, in sequence, in order for the trigger circuit to detect a true trigger. For instance, the arm point can enable a trigger comparator and the fire point can cause an output from the trigger comparator, signalling a true trigger. Generally, the hysteresis is set such that typical signal noise and internal instrument noise will not cause a false trigger. The size of the hysteresis determines the minimum signal necessary to produce a triggered waveform on the oscilloscope display screen.
FIG. 1 shows an input signal having an arm point and a fire point defining the triggering hysteresis, where the hysteresis is the fire point value minus the arm point value. FIG. 2 shows an input signal having a false trigger due to noise on the signal. FIG. 3 shows an example of the double trigger display which is due to a false trigger.
FIG. 4 shows a typical oscilloscope circuit wherein input signal 50 is buffered by buffer amplifier 110 and then sent to both a vertical gain vernier circuit 75 and trigger circuit 100. Vertical gain circuit 75 comprises a vertical amplifier 120 which controls the gain vernier along the vertical axis, that is, the voltage amplitude axis, of the oscilloscope display screen. Vertical amplifier 120 has a user-adjustable vertical gain vernier control 125, typically having a range of 1 through 2.5. Trigger circuit 100 comprises a trigger amplifier 130 and a trigger comparator 140. One input to trigger comparator 140 is the output of trigger amplifier 130. The other input to trigger comparator 140 is a user-adjustable trigger level input having trigger level offset control 145. Ideally, the trigger level input defines the voltage level at which the oscilloscope will trigger. Trigger comparator 140 will produce an output given a pre-determined difference between its inputs. Ideally, the output of trigger comparator 140 indicates a true trigger.
Typically, trigger comparator 140 has a fixed hysteresis. Typically, trigger amplifier 130 has a gain control 135. Varying the gain of trigger amplifier 130 varies the input to trigger comparator 140 and consequently the effective hysteresis is varied. For instance, with a fixed hysteresis on trigger comparator 140, a doubling of the output of trigger amp 130 will halve the hysteresis and a halving of the output of trigger amplifier 130 will double the hysteresis. This variation in the effective hysteresis can be seen in FIGS. 5A and 5B. In FIG. 5A, input signal 25A has a noise spike on its down slope but the size of the hysteresis prevents a false trigger. In FIG. 5B, input signal 25B has twice the gain of input signal 25A of FIG. 5A. Although the hysteresis is the same in both FIGS. 5A and 5B, doubling the gain of signal 25A to produce signal 25B also doubles the size of the noise spike which then is big enough to cause a false trigger. Therefore, doubling the signal effectively halves the hysteresis and halving the signal would likewise effectively double the hysteresis.
To prevent the problem of variation in the effective size of the hysteresis, the gain control 135 of trigger amplifier 130 is typically fixed when the instrument is calibrated. However, fixing the trigger amplifier gain has negative side-effects. To keep a signal spanned across a constant number of divisions on the display screen as the vertical gain 125 is adjusted from X1 to X2.5, the input to trigger amplifier 130 would also have to vary from X1 to X2.5, causing the hysteresis to vary inversely, requiring trigger level control 145 to have sufficient range to cover the largest signal at the lowest vertical gain setting as well as sufficient resolution to cover the smallest signal at the largest vertical gain setting. The slew rate demands through the trigger circuit path would also be X2.5 higher than if the signal range was fixed.